Project Summary Pediatric asthma affects over 8% of youth and is a leading cause of morbidity. Adherence to inhaled corticosteroids (ICS) can prevent asthma-related morbidity, however, adolescents with asthma commonly take fewer than 50% of their prescribed doses of ICS when assessed objectively. There are efficacious behavioral health interventions for improving adherence to ICS that are delivered in-person. However, the fact that the majority of adolescent do not take their medication as prescribed signals a clear problem with scaling the solutions to meet the public health need. Recent advances in smartphones and mobile health technologies provide a ubiquitous platform that can circumvent scalability challenges while concurrently approximating the active ingredients of existing behavioral health interventions. During the first decade of development, smartphone interventions for adolescents have largely relied on static text based reminders to cue adherence. With the invention of mobile sensors that wirelessly track ICS adherence, new approaches to intervention are possible that more closely mirror efficacious in-person interventions. The aims of this application are to merge the efficacious behavior change strategies found in behavioral health interventions with scalable mHealth technologies through the development and refinement of the Responsive Asthma Care for Teens program (ReACT). ReACT intervention content will be developed through an iterative user-centered design process that includes 1) an advisory board of youth with asthma, a pediatric pulmonologist, and a behavioral health expert, 2) one-on-one interviews with our target users, and 3) crowdsourced feedback. In tandem, we will program ReACT algorithms to tailor intervention behavior. We will conduct a 6-month randomized controlled trial followed by a 6-month maintenance phase to test the feasibility, acceptability, utilization, and efficacy of ReACT. A sample of 110 adolescents with persistent asthma will be randomized to receive ReACT or a static mHealth medication reminder control. A mHealth control condition enhances rigor by being an active comparator, limiting potential confounding variables, and allowing for isolation of the JITAI design. Feasibility and usability data will be collected throughout the trial and will be determined by examining enrollment and attrition rates and usage statistics. We will conduct a mixed-methods assessment of ReACT acceptability at the end of the 6-month intervention. Efficacy outcomes include changes in objectively monitored adherence to ICS (primary outcome), lung function, asthma control, asthma-related quality of life, and asthma-related morbidity. Data to evaluate efficacy outcomes will be collected at baseline, 3-, 6- 9- and 12-month assessment visits. We will evaluate theoretical mechanisms of change by testing for changes in capability, opportunity, and motivation (COM-B) as mediators of intervention outcomes. If successful, this intervention offers excellent potential for wide spread dissemination to address a pressing public health problem.